The present invention relates to electric discharge machining methods for satinizing with a predetermined degree of roughness the surface of a work piece in roll or plate form, and more particularly the invention relates to an improved electric discharge machining method in which the feeding of an electrode is controlled to automatically adjust the working gap between the electrode and a work piece.
In the past, methods of satinizing for example the surface of strip steel rolling rolls, particularly cold rolling rolls are known in the art in which impressions are formed in the polished roll surfaces by throwing hard metal particles, e.g., shot or grit against the roll surfaces, and recently attempts have been made to perform this type of processing by means of electric discharge machining. As is well known, the electric discharge machining is a process in which an insulating liquid, e.g., kerosine is introduced into a narrow working gap between an electrode and a work piece and a pulse waveform voltage is periodically applied across the electrode and the work piece to cause discharge therebetween and thereby machine the surface of the work piece. By rotating the roll in a circumferential direction and simultaneously gradually moving the electrode in the direction of rotation of the roll while performing the discharge machining at the surface of the roll, it is possible to continuously subject the roll surface to satin finishing operation and thereby to form discharge impressions all over the roll surface.
The satin finish thusly given to the roll surface has many advantages in that as compared with the mechanical impressions formed by the blasting of metal particles, the difference in height between the depressions and protrusions is great, the resulting shape is not only far superior in uniformity but also not susceptible to the production method and the hardness of the roll used and the metal structure in the roll surface is hardened by the discharge thus making the roll best suited for use as a rolling roll.
On the other hand, with the ordinary electric discharge machining, e.g., boring operation, an electrode feed control method is used in which in order to always maintain constant the average value of the voltage at the working gap, the voltage at the working gap is compared with a predetermined reference voltage and the difference therebetween is used to control a drive unit comprising a motor, hydraulic unit or the like, and consequently the working gap will be caused to repeatedly vary on both sides of a reference value in dependence on the frequency response characteristic of the drive unit. If this control method is applied to the machining of revolving rolls to which the present invention relates, due to the oscillation or hunting of the working gap and the resulting nonuniformity of the discharge condition, the amount of machining will be caused to change with time thus causing a streaky pattern in the surface of the roll. Of course, this change in the amount of machining is so small that the effect of the change on the worked surface cannot be perceived by the naked eye and the effect can be seen only by means of a chalk test effected by applying chalk powder to the roll surface or a test employing an oil stone. However, if the roll is used as a strip steel rolling roll for final rolling purposes, the presence of such a streaky pattern tends to produce ill effects on the rolled strip steel which in turn will be rejected as off-grade product.